Part of another set.

A is a subset of B when every member of A is a member of B.

Example: B = {1,2,3,4,5}

Then A = {1,2,3} is a subset of B

Other subsets of B include {2,3} or {1,4,5} or {4} etc...

But {1,2,6} is NOT a subset of B as it has 6 (which is not in B)

See: Set